lkml.org 
[lkml]   [2017]   [Sep]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 14/15] futex: convert futex_pi_state.refcount to refcount_t
    On Fri, 1 Sep 2017, Peter Zijlstra wrote:
    > On Fri, Sep 01, 2017 at 09:39:50AM +0200, Thomas Gleixner wrote:
    > > On Wed, 30 Aug 2017, Elena Reshetova wrote:
    > > > atomic_t variables are currently used to implement reference
    > > > counters with the following properties:
    > > > - counter is initialized to 1 using atomic_set()
    > > > - a resource is freed upon counter reaching zero
    > > > - once counter reaches zero, its further
    > > > increments aren't allowed
    > > > - counter schema uses basic atomic operations
    > > > (set, inc, inc_not_zero, dec_and_test, etc.)
    > > >
    > > > Such atomic variables should be converted to a newly provided
    > > > refcount_t type and API that prevents accidental counter overflows
    > > > and underflows. This is important since overflows and underflows
    > > > can lead to use-after-free situation and be exploitable.
    > > >
    > > > The variable futex_pi_state.refcount is used as pure
    > > > reference counter. Convert it to refcount_t and fix up
    > > > the operations.
    > > >
    > > > Suggested-by: Kees Cook <keescook@chromium.org>
    > > > Reviewed-by: David Windsor <dwindsor@gmail.com>
    > > > Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
    > > > Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
    > >
    > > Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
    >
    > So the thing to be careful with for things like futex and some of the
    > other core kernel code is the memory ordering.
    >
    > atomic_dec_and_test() provides a full smp_mb() before and after,
    > refcount_dec_and_test() only provides release semantics.
    >
    > This is typically sufficient, and I would argue that if we rely on more
    > than that, there _should_ be a comment, however reality isn't always as
    > nice.
    >
    > That said, I think this conversion is OK, pi_state->refcount isn't
    > relied upon to provide additional memory ordering above and beyond what
    > refcounting requires.

    So the changelogs should reflect that. The current one suggests that this
    is a one to one replacement for atomic_t just with the extra sanity checks
    added.

    Thanks,

    tglx

    \
     
     \ /
      Last update: 2017-09-01 11:44    [W:3.428 / U:0.268 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site